The present invention relates to water-tube boilers such as once-through boilers, natural circulation water-tube boilers and forced circulation water-tube boilers.
The water-tube boiler includes body of which is made up by water tubes. The body arrangement of such a water-tube boiler is, for example, that a plurality of water tubes are arranged into an annular shape. In the water-tube boiler of this form, a cylindrical space surrounded by the annular water tube array is used as a combustion chamber. In such a water-tube boiler, heat transfer primarily by radiation is performed within the combustion chamber, and then heat transfer primarily by convection is done in the downstream of the combustion chamber.
In recent years, such water-tube boilers are also desired to be further reduced in NOx and CO. The reduction in NOx, as it stands now, is implemented by fitting low-NOx burners or exhaust-gas re-circulation equipment to the existing boiler bodies. The reduction in CO is implemented by adjusting the state of combustion of the combustion equipment. However, further reduction in NOx and reduction in CO are demanded in keeping up with growing recognitions of environmental issues.
An object of the invention is to achieve further reduction in NOx and reduction in CO with simple structures of the boiler body and the burner.
In order to achieve the above object, the present invention provides a water-tube boiler comprising: a first water tube array made up of a plurality of first water tubes arranged into an annular shape; a combustion chamber defined inside the first water tube array; a first opening defined at part of the first water tube array; a cooling water tube array made up of a plurality of cooling water tubes arranged into an annular shape in a zone within the combustion chamber where burning-reaction ongoing gas is present; gaps provided between adjacent cooling water tubes so as to permit the burning-reaction ongoing gas to flow through; and a burning-reaction continuing zone, where burning reaction is continuously effected, provided between the cooling water tube array and the first water tube array, whereby the burning-reaction ongoing gas generally uniformly contacts the individual cooling water tubes.
In an embodiment of the invention, the water-tube boiler is characterized in that among the gaps, a specified number of gaps confronting the first opening are closed.
In an embodiment of the invention, the water-tube boiler is characterized in that among the gaps, width of gaps closer to the first opening is smaller than width of gaps farther from the first opening.
In an embodiment of the invention, the water-tube boiler is characterized in that a burner directed toward the combustion chamber is decentered from the center of the cooling water tube array so as to be away from the first opening.
In an embodiment of the invention, the water-tube boiler is characterized in that axis line of a burner directed toward the combustion chamber is tilted so as to be away from the first opening.
In an embodiment of the invention, the water-tube boiler is characterized in that the cooling water tube array is made up of a plurality of water tube arrays.
Further, in an embodiment of the invention, the water-tube boiler further comprises: a second water tube array made up of a plurality of second water tubes arranged into an annular shape outside the first water tube array; a second opening defined at part of the second water tube array; and a gas flow passage provided between the first water tube array and the second water tube array.
The present invention is embodied as a water-tube boiler of the multiple-tube type. Further, the water-tube boiler of the present invention is applied not only as steam boilers or hot water boilers, but also as heat medium boilers in which a heat medium is heated.
A first water tube array is made up by arranging the plurality of first water tubes into an annular shape, and a combustion chamber is defined inside this first water tube array. A first opening is provided at part of the first water tube array. This first opening may be provided as a single opening having an appropriate width in the circumferential direction, or as a plurality of openings divisionally by interveniently providing one or two first water tubes. A cooling water tube array is made up of a plurality of cooling water tubes arranged into an annular shape, in a zone within the combustion chamber where burning-reaction ongoing gas is present. Gaps are provided between adjacent cooling water tubes so as to permit the burning-reaction ongoing gas to flow through. The burning-reaction ongoing gas includes a flame, being a high-temperature gas under progress of burning reaction. That is, the cooling water tubes are placed within the flame, thus being in contact with the flame. Between the cooling water tube array and the first water tube array, a zone where burning reaction is continuously effected is provided.
In the combustion chamber, the burning-reaction ongoing gas tends to flow toward the first opening, causing a tendency that a larger amount of burning-reaction ongoing gas that contacts cooling water tubes located closer to the first opening while a smaller amount of burning-reaction ongoing gas that contacts cooling water tubes located farther from the first opening. However, the water-tube boiler of this invention is so constituted that the burning-reaction ongoing gas generally uniformly contacts the cooling water tubes in the following manner.
First, contrivance for the arrangement of the cooling water tubes is explained. Out of the gaps between the cooling water tubes, a specified number of gaps confronting the first opening are closed. Also among the gaps between the cooling water tubes, width of gaps closer to the first opening is smaller than width of gaps farther from the first opening. By these arrangements, the burning-reaction ongoing gas is inhibited from flowing short toward the first opening, so that the burning-reaction ongoing gas generally uniformly contacts the individual cooling water tubes.
Next, contrivance for the arrangement of the burner provided so as to be directed toward the combustion chamber is explained. The burner is decentered from the center of the cooling water tube array so as to be away from the first opening. Also, the axis line of the burner is tilted so as to be away from the first opening. By these arrangements, the burning-reaction ongoing gas is inhibited from expanding unevenly due to the arrangement of the cooling water tubes, so that the burning-reaction ongoing gas generally uniformly contacts the individual cooling water tubes.
Flow and reaction of the burning-reaction ongoing gas within the combustion chamber are explained in detail. Burning-reaction ongoing gas that has been generated by the fuel burning in the combustion chamber is cooled by the cooling water tubes, with the temperature lowered, by which the generation of thermal NOx is suppressed. The burning-reaction ongoing gas, which flows through the gaps between the cooling water tubes, contacts the overall surfaces of the cooling water tubes, thus being cooled. As can be explained for Zeldovich mechanism, the higher the temperature of burning reaction, the higher the generation rate of thermal NOx increases considerably; the lower the temperature of burning reaction, the lower the generation rate of thermal NOx, where the generation rate of thermal NOx is considerably lower when the temperature of burning reaction is 1400.degree. C. or lower. Therefore, number and heat transfer area of the cooling water tubes are set in order that the temperature of burning reaction becomes 1400.degree. C. or lower. When the cooling water tube array is made up of a plurality of water tube arrays, the heat transfer area per unit space is increased so that NOx reduction effect by cooling is improved.
The burning-reaction ongoing gas that has passed through the gaps between the cooling water tubes continues burning reaction in a zone between the cooling water tube arrays and the first water tube array, where burning reactions of intermediate products of burning reactions such as CO and HC and unburnt components of the fuel are continuously effected. Since CO remaining in the burning-reaction ongoing gas is oxidized into CO.sub.2, the amount of CO emission from the boiler is reduced.
As described above, by the arrangement that the burning-reaction ongoing gas generally uniformly contacts the individual cooling water tubes, the NOx reduction effect by cooling can be obtained generally uniformly at the individual cooling water tubes. Therefore, increases in NOx due to insufficient cooling and increases in CO due to excessive cooling, which would occur upon the occurrence of variations in cooling, can be prevented.
It is preferable, depending on the circumstances of embodiment, that a second water tube array is provided by arranging a plurality of second water tubes. A gas flow passage is defined between the first water tube array and the second water tube array, and a second opening is provided at part of the second water tube array. This second opening may be provided as a single opening or a plurality of openings, like the first opening. Within the combustion chamber, radiant heat transfer and convective heat transfer are effected. The gas that has nearly completed the burning reaction flows into the gas flow passage through the first opening, where convective heat transfer is primarily effected in the gas flow passage. By providing the second water tube array, the amount of heat transfer can be increased. The burning-reaction completed gas is exhausted outside through the second opening.